A conventional magnetic disk 4 is fixed to a spindle motor 1, as shown in FIG. 4(a), by fitting a disk hole 5 of the magnetic disk 4 to a hub 3 coupled to the rotary shaft of the spindle motor 1, holding the magnetic disk 4 from the upper side with a circular disk clamp 6 made of metal such as stainless steel, and screwing down a screw bolt 7 to depress the magnetic disk 4 onto a flange face 2 of the rotary shaft. The disk clamp 6 has a curved peripheral portion 6a which works as a plate spring. The curved peripheral portion 6a elastically depresses the portion of the magnetic disk 4 surrounding the disk hole 5 to fix the magnetic disk 4 vertically. The magnetic disk 4 is also fixed horizontally by the friction between the disk 4 and the flange face 2 corresponding to the depressing force.
The fixing method described above, which uses the disk clamp 6 and screw bolt 7, requires tremendous depressing force to obtain a sufficient frictional force to securely fix the magnetic disk in its radial and circumferential directions. Since the peripheral area of the disk 4 surrounding the disk hole 5 (herein after referred to as clamped area) is strongly clamped by the flange face 2 and the curved peripheral portion 6a of the disk clamp 6 as shown in FIG. 4b), strain deformation spreads to an adjacent area around the clamped area to form a deformed area. The deformed area causes an output difference between a pair of magnetic heads (not shown), facing opposed to one another, through the magnetic disk 4. The deformed area does not cause a serious problem in the disk storage device for a disk having a diameter of 3.5 inches or more in which the data storage area is separated from the clamped area. However, it is impossible to store and readout data in and from the deformed area in the disk storage device for a disk having a diameter of 2.5 inches or less in which the clamped area extends near to the data storage area.
Using a weaker depressing force may eliminate the deformed area. A weaker depressing force, however, causes deterioration of the disk's anti-mechanical-shock property since the weaker depressing force reduces the friction on the disk plane and allows the magnetic disk to displace horizontally.
To solve the above described problem, a disk storage device as shown in FIG. 5(a) is proposed in the Japanese Patent Application No. H04-178715. In the proposed disk storage device of FIG. 5(a), the magnetic disk 4 is fixed to the spindle motor 1 by fitting the disk hole 5 of magnetic disk 4 to the hub 3 coupled to the rotary shaft of the spindle motor 1, fitting an elastic member 8 with high coefficient of friction around the hub 3 and onto the fitted magnetic disk 4, holding the magnetic disk 4 and the elastic member 8 from the upper side with a circular disk clamp 9 made of metal such as stainless steel, and screwing down a screw bolt 7 to depress the magnetic disk 4 onto a flange face 2. The elastic member 8 facilitates attempting fixing the magnetic disk 4 without causing any deformed area as shown in FIG. 5(b), since the elastic member 8 depresses the magnetic disk 4 to fix the magnetic disk 4 elastically and uniformly and increases the frictional force. However, a deformed area of a small diameter is still formed around the clamped area, for example of 1.89 inches or less, since a depressing force is exerted to the magnetic disk even when the elastic member 8 is installed. Especially when a thin base plate of the magnetic disk of small diameter is made of aluminum alloy, glass or ceramics, the deformed area is caused so often that reduction of the data storage area can no longer be ignored.
In view of the foregoing, an object of the present invention is to provide a disk storage device in which a magnetic disk is fixed in a manner that is free from deformation, secures a wide effective data storage area and shows excellent anti-mechanical shock property.